The Cardiovascular Research Program is involved in studies of myocardial physiology and the dynamics of coronary blood flow. The mechanisms underlying development of force in heart muscle are being explored using new methods of direct microscopic visualization to study sarcomere dynamics. Light diffraction methods have been combined with video and filming techniques and are being used to control mechanical activity to provide insight into such properties of cardiac muscle as series elasticity and passive stiffness. Excitation-contraction coupling is being studied using agents which alter either intracellular or trans-sarcolemmal calcium fluxes. The inter-relations between coronary blood flow, its distribution and collateral blood flow on myocardial performance and tissue survival during infarction are being studied. The effects of mannitol and nitroglycerin on these factors are being specifically explored in new quantitative models.